Bright silver plating bath and process



United States Patent 3,219,558 BRIGHT SILVER PLATING BATH AND PROCESS Donald Gardner Foulke, Watchung, Plainfield, N.J.,

assignor to Sel-Rex Corporation, Nutley, N .J., a corporation of New Jersey No Drawing. Filed Mar. 22, 1962, Ser. No. 181,793 4 Claims. (Cl. 204-46) This invention is concerned with the electrodeposition of silver and primarily with the electrodeposition of bright silver.

The electrodeposition of bright silver has economic advantages in that bufiing is not necessary after plating which results in savings in time and in silver as well in that the usual bufling losses are eliminated.

This invention is based on the discovery that antimony in combination with an alkali metal salt of methylene bis naphthalene sulfonic acid, when added to a conventional silver plating solution makes it possible for one to obtain fully bright deposits over a wide range of plating conditions. The method of addition of the antimony is relatively unimportant. However, halides are not particularly desirable in a silver bath, so the next most common antimony compounds, tartar emetic (potassium automonyl tartrate) and antimony oxide are considered most suitable from the standpoint of availability.

A typical bath, showing the possible variations in concentration, is shown in Table I.

The potassium salts are most suitable although sodium and lithium salts may be substituted. The presence of free caustic soda or potassium hydroxide militates against the addition of ammonium salts.

Previously disclosed bright silver processes have employed antimony as a brightener. In one instance, it has beenistipulated that considerable alkali tartrate must be present in the bath and in another, it was stipulated that tartar emetic was unsuitable and that the antimony had to be present in the bath in a very complexed form. Ac cording to the process of this invention excess tartrates are not necessary and a simple antimony compound in conjunction with methylene bis naphthalene sulfonic acid operates to give exceptionally bright deposits. Complex antimony compounds may be employed, however. This simple bath is much less apt to lose its brightening power during operation of the process.

The baths may be operated at current densities of 0.1- 5.0 amperes per square decimeter.

There appears to be a synergistic action with the two additives. The simple silver bath containing antimony trioxide, antimony potassium tartrate or even the complex antimony ethylene diamine tetraacetic acid or antimony nitriloacetate (which is another method for introducing antimony into the bath) does not produce full bright deposits over wide current density ranges. The simple silver bath containing methylene bis naphthalene sulfonic acid sodium salt, produces hazy deposits. However, the presence of both ingredients results in a surprisingly bright electrodeposit.

In order for the bath to yield the brightest deposits over a wide range of current densities it is necessary to maintain an alkalinity equivalent to 5 grams per liter of caustic potash, preferably at 7.5 to 8 grams per liter. At these concentrations, the antimony is kept in solution in the bath.

A number of other addition agents were tried in conjunction with the preferred combination of antimony and naphthalene bis sulfonic acid, sodium salt, including butynediol, ethoxylated butynediol, butenediol ethylene glycol, with no noticeable improvement. As a matter of fact, these as well as selenium compounds and thiocarbamates tend to produce a haze, apparently interfering with the synergistic effect of the antimony-methylene bis sodium naphthalene sulfonate combination.

Specific examples of some of the baths successfully employed to give bright sliver deposits to follow will indicate the broad nature of this discovery and emphasize the importance of the presence of both brighteners in the bath.

It should be stated that a suitable electrolyte for silver plating consists of a soluble silver cyanide and excess alkali cyanide or alternately a silver salt is added to a bath containing excess alkali cyanide resulting in the formation in situ of the complex silver cyanide. Cyanide silver plating solutions may vary over rather wide ranges of complex silver cyanide and alkali cyanide. Table I discloses the preferred composition, but for special cases the range may be extended much above and below the optimum. Of course, it is customary toadd carbonates and hydroxides to increase the conductivity and for other reasons. These salts are used in the bright plating process herein described but in no way should they be considered essential to the working of the process, except, as stated previously, the presence of free alkali is helpful in maintaining the antimony in solution.

Example 1 To a silver bath containing 34 g./l. of silver as potassium silver cyanide and g./l. of potassium cyanide along with 15 g./l. of potassium hydroxide was added 0.2 g./l. of antimony as antimony tartrate. The panel was semi-bright to 2.0 amp./dm. whereas a panel plated in the plain silver bath was semi-bright to 1.3 amp./dm.

Increasing the antimony to 0.4 g./l. improved the brightness and extended the range to 4 amp/rim The addition of 1.2 g./l. of methylene bis sulfonic acid, sodium salt, eliminated all haze producing a brilliant deposit from essentially 0 to 4.0 amp./dm.

Example 2 With a basic bath of the same composition as in Example I, 0.006 g./l. of the sodium salt of methylene bis naphthalene sulfonic acid produced a panel with a semibright range (to 0.8 amp/dm?) less than that obtained in the bath without the organic sulfonate. Additional methylene bis sulfonic acid, sodium salt, decreased rather than improved the range, showing a semi-bright range of up to only 0.3 amp./dm. with 12 g./l. However, when 0.4 g./l. of antimony was added a full-bright, haze-free deposit was obtained.

Example 3 To the same basic silver hath given in Example 1 was added 3 g./l. of antimony and 4 g./l. of methylene bis sulfonic acid, sodium salt, whereupon a brilliant deposit, from 0 to 4.0 amp/dim was obtained. The addition of another 0.5 g./l. of naphthalene bis sulfonic acid, sodium salt, had no etfect and it appears there is no practical upper limit for this compound other than economies which dictate using as little as necessary to get full brightness. However, if one adds more than 3 g./l. of antimony the high current density range will show darkening but still bright, areas.

Example 4 To the standard silver solution (of Example 1, for example) was added 1 g./l. of sodium methylene bis naphthalene sulfonate and 0.2 -g./ l. of antimonous oxide. A brilliant deposit was obtained with a range up to about 3.5 amperes/dmfi.

Example 5 To the standard silver solution of Example 1 was added 1 g./ 1. of methylene bis naphthalene sulfonate and 0.2 g./l. of antimony chloride. Some precipitation occurred, most of which redissolved. A Hull cell panel plated in the bath shows a bright range from 03.5 amperes/dmfi.

I claim: 1. An aqueous bath for the deposition of silver consisting essentially of from about 7.5 g./l. to about 112 g./l. of a soluble silver cyanide, from about 25 g./l. to 450 g./l. of a soluble cyanide, from about .006 to about 38 g./l. of a soluble salt of methylene bis naphthalene sulfonic acid and from about 0.25 g./l. to about 12 g./ 1. of antimony added as a compound soluble in the bath. 2. An aqueous bath as claimed in claim 1 in which the antimony compound is tartar emetic.

3. An aqueous bath as claimed in claim 1 in which the antimony compound is antimony oxide.

4. A process for the deposition of silver consisting of immersing a conductive workpiece in a solution consisting essentially of about 7.5 g./1. to about 112 g./l. of a soluble alkali silver cyanide, about 25 g./l. to about 450 g./l. of a soluble alkali cyanide, about 0.25 g./l. to about 12 g./l. of antimony added as a soluble antimony compound and about .006 to about 38 g./l. of a soluble salt of methylene bis naphthalene sulfonic acid, and

passing an electric current through the bath in such a fashion that the workpiece is the cathode and the current density thereon is about 0.1-5 .0 amperes per square decimeter.

References Cited by the Examiner UNITED STATES PATENTS 2,701,234 2/1955 Wernlund 20452.1 2,735,808 2/1956 Greenspan 20446 2,771,411 11/1956 Chester et al. 20455.1 2,777,810 1/ 1957 Ostrow 20446 2,787,590 4/1957 Rinker 204 55.1

JOHN H. MACK, Primary Examiner.

JOHN R. SPECK, Examiner. 

1. AN AQUEOUS BATH FOR THE DEPOSITION OF SILVER CONSISTING ESSENTIALLY OF FROM ABOUT 7.5 G/L. TO ABOUT 112 G./L. OF A SOLUBLE SILVER CYANIDE, FROM ABOUT 25 G./L. TO 450 G./L. OF A SOLUBLE CYANIDE, FROM ABOUT .006 TO ABOUT 38 G./L. OF A SOLUBLE SALT OF METHYLENE BIS MAPHTHALENE SULFONIC ACID AND FROM ABOUT 0.25 G./L. TO ABOUT 12 G./L. OF ANTIMONY ADDED AS A COMPOUND SOLUBLE IN THE BATH. 